A spark plug described in Patent Document 1 has been heretofore known. The spark plug has a cylindrical metal shell. An insulator, which is formed cylindrically by a through-hole so as to extend in the axial direction of the metal shell, is fixed to the inside of the metal shell. A center electrode and a terminal attachment are provided in the inside of the metal shell and the insulator. The center electrode extends in the axial direction of the metal shell and has an electrically dischargeable front end protruding from a front end of the insulator, and a rear end fixed into the through-hole. The terminal attachment extends in the axial direction of the metal shell and has a rear end protruding from the rear end of the insulator, and a front end fixed into the through-hole. An end of a ground electrode is fixed to the metal shell so that a spark discharge gap is formed between the ground electrode and the center electrode.
The spark plug has an electrically conductive connection layer which is provided in the through-hole of the insulator and between the center electrode and the terminal attachment so that the center electrode and the terminal attachment are electrically connected to each other by the electrically conductive connection layer. The electrically conductive connection layer has a first electrically conductive sealing layer, a resistor and a second electrically conductive sealing layer which are arranged in order when viewed from the center electrode side. There is description that each of the first and second electrically conductive sealing layers is made of electrically conductive glass which contains a glass component, and a metal component, and that Cu can be used as an example of the metal component. Besides the spark plug configured as described above, a spark plug having an electrically conductive connection layer made of a combination of an electrically conductive sealing layer and a resistor arranged in order viewed from the center electrode side and a spark plug having an electrically conductive connection layer made of only an electrically conductive sealing layer have been known.
When this type spark plug is mounted in an engine and a high voltage is applied between the metal shell and the terminal attachment, electric discharge is generated in the spark discharge gap between the center electrode and the ground electrode to thereby perform ignition at the time of driving the engine. On this occasion, in the spark plug in which Cu is used as an example of the metal component of the electrically conductive glass in the electrically conductive sealing layers (first and second electrically conducive sealing layers described in Patent Document 1), the terminal attachment and the center electrode are fixed to the insulator while airtightness is kept by the glass component. In the spark plug, contact resistance between each of the terminal attachment and the center electrode and the electrically conductive connection layer is reduced by Cu to thereby keep excellent electric conductivity therebetween.
[Patent Document 1]
Japanese Patent Laid-Open No. 127530/1977